Open access peer-reviewed chapter

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes

Written By

Hitoshi Ishida

Submitted: 30 April 2017 Reviewed: 12 February 2018 Published: 21 March 2018

DOI: 10.5772/intechopen.75199

From the Edited Volume

Carbon Dioxide Chemistry, Capture and Oil Recovery

Edited by Iyad Karamé, Janah Shaya and Hassan Srour

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Conversion of CO2 into useful chemicals is attractive as a solution of the fossil fuel shortage and the global warming problems. Reduction of CO2 into carbon monoxide (CO) and formic acid (HCOOH) is also important for obtaining the materials in organic syntheses. There are a lot of studies on the catalysts for electrochemical/photochemical CO2 reduction. Especially, transition metal complexes have actively researched as the molecular catalysts for CO2 reduction. In this chapter, the electrochemical/photochemical CO2 reduction catalyzed by cis-[Ru(bpy)2(CO)2]2+ (bpy: 2,2′-bipyridine) and trans(Cl)-[Ru(bpy)(CO)2Cl2] is described as a representative example.


  • CO2 reduction
  • artificial photosynthesis
  • electrochemistry
  • photochemistry
  • ruthenium

1. Introduction

Utilization of CO2 becomes more and more important with increasing CO2 emission which causes the global warming and the ocean acidification problems [1, 2]. The huge CO2 emission also relates on depletion of fossil fuels. The conversion of CO2 into useful fuels and chemicals is very urgent to solve the abovementioned problems. The use of biomass instead of fossil fuels is actively researched and partly undertaken [3]. In many chemical laboratories, fixation of CO2 into organic compounds by organometallic catalysts is vigorously studied [4].

Reduction of CO2 with electrons is an attractive chemical conversion to obtain the useful products for fuels and chemical materials. It is so simple that it can be applied to photocatalyses which supply electrons from electron donors such as water. The equilibrium potentials (E0’ V vs. SHE at pH 7) for CO2 reduction are listed in Figure 1 [5, 6]; they are thermodynamic values and tend to positively shift with increasing the numbers of electrons participated. One-electron reduction of CO2 requires very high energy. Furthermore, the product, CO2 anion radical (CO2∙), is difficult to give useful organic chemicals because it is a very strong reducing reagent to reduce other molecules and recover CO2. Thus, the CO2 reductions with multielectrons are desired; however, their reactions are generally difficult even in the electrochemical reduction. A reason is that the intermediates would release from the surface of the electrode as the products before accepting further electrons. To achieve CO2 reduction with more than two electrons, the catalysts which allow to lower the activation energies are required. In other words, the catalysts can undergo the CO2 reduction at the potentials closed to the equilibrium ones. The two-electron reduction of CO2 produces carbon monoxide (CO) and formic acid (HCOOH). The equilibrium potentials are more negative than the proton reduction to afford H2. Therefore, the catalysts which can selectively reduce CO2 rather than H+ are also desired. Both CO and HCOOH are useful chemicals: CO can be converted into liquid hydrocarbons by using the Fischer-Tropsch reaction [7], and HCOOH which can be readily converted to H2 is a safe storage material for H2 [8].

Figure 1.

Equilibrium potentials for CO2 reduction (E0 V vs. SHE (pH 7)).

A lot of metal complexes have been researched for the CO2 reduction catalyses [9, 10, 11, 12, 13, 14, 15, 16]. Until now, the metal complexes of Mn [17, 18, 19], Fe [20, 21], Co [22, 23, 24], Ni [24, 25, 26, 27, 28], Cu [29], Mo [30], Ru [31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64], Rh [65, 66], Pd [67, 68], W [30], Re [69, 70, 71, 72, 73, 74, 75, 76], Os [77, 78], Ir [65, 66, 79, 80] have been reported as the catalysts for CO2 reduction. Figure 2A shows the elements of the metal complexes acting as the electrochemical CO2 reduction catalysts. The metal complexes indicated in red include the catalysts for photochemical reduction. Figure 2B shows the examples of the metal complexes as the CO2 reduction catalysts. These catalysts based on metal complexes are sometimes called as “molecular catalysts” because they can be designed on the molecular levels by selecting the metal elements and the ligands. The representative and efficient catalysts for CO2 reduction are nickel(II) cyclam (cyclam: 1,4,8,11-tetraazacyclotetradecane), ruthenium(II) polypyridyl carbonyl complexes and rhenium(I) bipyridyl tricarbonyl complexes. Recently, the complexes with nonprecious metals such as manganese(II) and iron(II) attract much attention. They are abundant and readily available, while they are less durable and efficient as the disadvantageous points.

Figure 2.

Metal complexes reported as CO2 reduction catalysts: (A) the metal elements in the complexes (the elements in the metal complexes for photocatalyses are indicated in red) and (B) the molecular structures.

In this chapter, the electrochemical CO2 reduction catalyzed by the ruthenium complexes as the examples is described. The reduction products are CO and formic acid, while the nickel and rhenium complexes selectively yield CO. Discussion for the catalytic mechanisms is introduced particularly for the factors determining the product selectivity. In the next section, the photocatalytic CO2 reduction assisting by the photosensitizers is described. The reaction procedures, the principles for selecting the photosensitizers and the electron donors, and the photocatalytic mechanisms are summarized. Furthermore, application of the homogeneous catalytic systems to heterogeneous catalyses, which is practically advantageous in the viewpoints of separation of the catalysts from the reactants and the products, is described. In the final section, the artificial photosynthetic systems, which would be realized by utilizing the molecular catalysts, are prospected.


2. Electrochemical CO2 reduction

The representative molecular catalysts based on ruthenium complexes are cis-[Ru(bpy)2(CO)2]2+ (bpy: 2,2′-bipyridine), trans(Cl)-[Ru(bpy)(CO)2Cl2] and the derivatives (Figure 3). They have the bipyridyl ligand which would act as an electron reservoir. The efficient catalysts have the carbonyl ligand, which would draw electrons. It tends to lower the reduction potentials of the metal complexes as well as to lower the overpotentials for the CO2 reduction.

Figure 3.

Ruthenium-bipyridyl complexes as electrochemical CO2 reduction catalysts: (A) cis-[Ru(bpy)2(CO)2]2+ and (B) trans(cl)-[Ru(bpy)(CO)2Cl2].

2.1. Electrochemical analysis

Electrochemical analyses (e.g., cyclic voltammetric measurements) are recommended to know the electrochemical properties of the molecular catalysts. The analyses do not only teach us the reduction potentials of the metal complexes but also show whether the complexes can react with CO2 or not. Figure 4 shows the cyclic voltammograms (CVs) of cis-[Ru(bpy)2(CO)2]2+ in CH3CN or CH3CN/H2O (9:1). The Ag-Ag+ (CH3CN) reference electrode (0.10 M Tetrabutylammonium perchlorate (TBAP) /0.01 M AgNO3 in CH3CN) is used; the potential (0.00 V vs. Ag/AgNO3 (CH3CN)) corresponds to −0.09 V vs. Fc/Fc+ in CH3CN. The CV of cis-[Ru(bpy)2(CO)2]2+ in CH3CN under Ar shows an irreversible reduction wave at −1.3 V vs. Ag-Ag+ (CH3CN) as shown in Figure 4 (black line). The irreversible reduction suggests that the one-electron reduction accompanies with a chemical reaction followed by further one-electron reduction. Such a reaction mechanism is called as electrochemical-chemical-electrochemical (ECE) one. The CV under CO2 is a little different from that under Ar, suggesting that the reduced species react with CO2 (Figure 4, blue line). In CH3CN/H2O (9:1), the CV exhibits a strong cathodic current under CO2 (Figure 4, red line), which corresponds to the catalytic reduction of CO2 in the presence of a proton source such as water. The catalytic reduction currents can be analyzed to estimate the efficiency of the catalyst [20, 81]; however, it should be noted that the cathodic currents do not always exhibit the catalytic CO2 reduction [82]. The electrolyses of the metal complexes under CO2 should be carried out to confirm the catalytic efficiency.

Figure 4.

Cyclic voltammograms of cis-[Ru(bpy)2(CO)2](PF6)2: In Ar (black) or CO2-saturated CH3CN (blue) or in CO2-saturated CH3CN/H2O (9:1) (red) containing NBu4ClO4 (0.10 M).

2.2. Electrolysis

A typical electrolysis cell is shown in Figure 5. The cell for reduction (the side of the working electrode) is separated from the cell for oxidation (the counter electrode) with a membrane such as Nafion. A glassy carbon or a Pt plate is used for the electrodes. The metal complex is dissolved in the reaction solution and acts as the homogenous catalyst. CO2 is bubbled with a needle through the septum before electrolysis. Electrochemical CO2 is carried out in batch mode. Reduction of CO2 occurs on the working electrode at the electrochemical cell. Sampling of the gaseous and liquid phases is performed by a syringe through the septum. The gaseous products (CO and H2) are analyzed by gas chromatography. The liquid product, HCOOH, is analyzed by electrophoresis, ion chromatography or gas chromatography. The electrolysis is carried out by the controlled potential method, where the potential is determined from the electrochemical analysis (e.g., CVs). The chronopotentiometry, in which the current is constant during the electrolysis, is important for the industrial use. However, the results in the constant potential lead to elucidate the catalyses because the electrolysis potential relates on the catalytic species. Thus, almost all the scientific researches adopt the controlled potential electrolyses.

Figure 5.

Electrolysis cell for electrochemical CO2 reduction.

2.3. Electrocatalytic CO2 reduction by cis-[Ru(bpy)2(CO)2]2+

The ruthenium complexes are used as the homogeneous catalysts by dissolving in the reaction solution. The electrolysis of the CO2-saturated H2O/DMF (1:1) solution of cis-[Ru(bpy)2(CO)2]2+ was carried out at −1.50 V vs. SCE with an Hg pool as the working electrode (Figure 6) [62]. The catalyst could selectively reduce CO2 to afford CO and HCOOH, while H2, the reduction product of water, scarcely evolved. As the reaction proceeded, the speed for CO production got slow, but HCOOH production became fast. It was interpreted as the result of the decreasing the proton concentration ([H+]) in the reaction solution by consumption of the proton during the reduction. Actually, the reactions in the buffered solution with H3PO4-NaOH exhibited that the production speeds of the CO2 reduction were unchanged during the reactions. It was the decisive result that HCOOH selectively produced when phenol with the high pKa (ca. 9.95) was used as the proton source. These results suggest that there is an acid-base equilibrium between two intermediates in which one is for CO production and another for HCOOH.

Figure 6.

Plots of the amounts of products vs. the electricity in the electrolysis (−1.50 V vs. SCE) of CO2-saturated H2O/DMF (1:1 v/v) solution containing cis-[Ru(bpy)2(CO)2](PF6)2 (5.0 × 10−4 M) and LiCl (0.10 M) as the supporting electrolyte at room temperature.

Thus, the mechanism involving the equilibrium among the carbonyl complex [Ru(bpy)2(CO)2]2+, the carboxylic acid complex [Ru(bpy)2(CO)(C(O)OH)]+ and the CO2 adduct complex [Ru(bpy)2(CO)(CO2)] was proposed for the catalytic CO2 reduction (Figure 7, left cycle) [58, 62]. All the complexes were isolated, and the crystal structures were characterized [83]. In the mechanism, [Ru(bpy)2(CO)2]2+ is reduced to yield the coordinated unsaturated species [Ru(bpy)2(CO)] with evolving CO. The five coordinated complex reacts with CO2 to afford the η1-CO2 adduct complex, [Ru(bpy)2(CO)(CO2)], in which CO2 coordinates to the metal center at the carbon atom. The electronic structure of the CO2 bound complex still remains unknown. In the original report [62], it is drawn as [Ru(bpy)2(CO)(COO)]+ in which as electron localizes on the CO2 ligand. In Figure 7, it is drawn as [Ru(bpy)2(CO)(CO2)]0 which is the resonance structure of [Ru(bpy)2(CO)(COO)]+. The CO2 adduct complex is protonated to give the carboxylic acid complex [Ru(bpy)2(CO)(C(O)OH)]+ and further protonated to recover the carbonyl complex [Ru(bpy)2(CO)2]2+. The carboxylic acid complex could be reduced to yield HCOOH, and the carbonyl complex to produce CO. The proposed idea reasonably elucidates the experimental results that the catalytic CO2 reduction gives CO and HCOOH under protic and less protic conditions, respectively. This idea is also supported by the result that the ruthenium complex derivatives give the CO/HCOO selectivity depending on the different equilibrium constants [58]. The carbonyl complex reacts with dimethylamine to afford the carbamoyl complex [Ru(bpy)2(CO)(C(O)N(CH3)2)]+, and the electrochemical CO2 reduction in the presence of dimethylamine produces N,N-dimethylformamide (DMF) [84]. It is also an evidence which the carbonyl complex would exist in the catalysis.

Figure 7.

Two proposed mechanisms for CO2 reduction catalyzed by cis-[Ru(bpy)2(CO)X]n+ (X = CO (n = 2); H (n = 1)): Metal-CO2 adduct mechanism and hydride mechanism.

On the other hand, the ruthenium hydride complex [Ru(bpy)2(CO)H]+ is known to react with CO2 to yield the formate complex [Ru(bpy)2(CO)(OC(O)H)]+ [85]. In the conversion, CO2 is inserted into the Ru-H bond. The formate complex can release formate ion (HCOO) and is considered to be an intermediate for HCOO production. Based on the results, the hydride mechanism is proposed (Figure 7, right cycle). In the mechanism, the coordinated unsaturated species [Ru(bpy)2(CO)] does not react with CO2 but a proton to yield the hydride complex. The hydride mechanism reasonably explains the CO2 reduction to produce HCOO. However, it has a couple of problems [16]. One is that the mechanism is difficult to elucidate the CO production. Production of HCOO may occur through the hydride mechanism, while CO may produce through the M-CO2 adduct mechanism. In this case, the product selectivity (CO/HCOO) should be controlled by the reactivity difference between CO and H+ with the coordinated unsaturated complex. Under the protic conditions, the selectivity of HCOO production should be enhanced; however, the selectivity of the catalyses gives the opposite tendency. Thus, the pH in the solution or the pKa value of the proton source dependence on the electrochemical CO2 reduction cannot be explained. Another is that the ruthenium catalyst does not evolve H2 so much in the CO2 reduction. It suggests that the catalyst intermediate strongly binds with CO2 rather than H+.

Nevertheless, the hydride mechanism is supported by many researchers. It is because there are many research works on the CO2 insertion into Metal-H bonds to afford the corresponding metal formate complexes. On the other hand, the research works on the carboxylic acid complex are fewer, and no mechanical pathways of HCOO production from the carboxylic acid complex are not understood on the molecular levels.

2.4. Electrocatalytic CO2 reduction by trans(cl)-[Ru(bpy)(CO)2Cl2]

Trans(Cl)-[Ru(bpy)(CO)2Cl2] is known to be an efficient catalyst for electrochemical CO2 reduction [58]. The catalytic activity and the product selectivity are similar as these of cis-[Ru(bpy)2(CO)2]2+. Reduction of trans(Cl)-[Ru(bpy)(CO)2Cl2] induces to release Cl ion to afford the coordinated unsaturated complex. This complex is considered to an intermediate which can bind with CO2; however, it induces polymerization in the absence of CO2 as shown in Figure 8 [86, 87]. The polymer with Ru(0)-Ru(0) bonds is also an efficient electrocatalyst for CO2 reduction [50, 54]. The complex is electrochemically reduced to polymerize on the cathode electrode. The electrode modified with the polymer is moved to another electric cell, and it works in the presence of CO2 as the active electrode for electrochemical CO2 reduction. Researches to make the modified electrode stable have been actively done: introduction of pyrrole groups to the bipyridyl ligand also yields pyrrole polymers to stabilize the ruthenium polymer.

Figure 8.

Electroreductive polymerization of trans(cl)-[Ru(bpy)(CO)2Cl2].

The catalytic reaction mechanisms are also unknown but are considered similar as these of cis-[Ru(bpy)2(CO)2]2+. Machan et al. reported the electrochemical CO2 reduction catalyzed by trans(Cl)-[Ru(6Mesbpy)(CO)2Cl2], which has two bulky groups at 6,6′-positions in 2,2′-bipyridine [35]. The complex does not polymerize because of the steric hindrance. They discussed the reaction mechanisms based on the hydride mechanism (Figure 9). The precursor complex is at the center of the scheme. It is reduced with releasing Cl ion to yield the coordinated unsaturated species, which does not bind with CO2 but H+ to afford the hydride complex. The hydride complex reacts with CO2 to yield the formate complex, which is reduced to produce HCOO with recovering the original complex. However, the catalyst mainly produces CO not HCOO. In the mechanism, the formate complex converts to the carboxylic acid complex and then the carbonyl complex by dehydration. The conversion of the formate complex to the carbonyl complex via the carboxylic acid complex is not known, and therefore further researches are expected.

Figure 9.

A proposed mechanism of electrochemical CO2 reduction catalyzed by trans(Cl)-[Ru(6Mesbpy)(CO)2Cl2].

Homogeneous catalysts are advantageous from the viewpoints of elucidating the catalytic reaction mechanisms compared to heterogeneous ones because the homogenous catalysts can be examined by using many spectroscopic techniques. Nevertheless, the mechanisms of the electrochemical CO2 reduction catalyzed by the ruthenium complexes still remain unknown. There may be potentially many intermediates and pathways in the catalyses, and they depend on the reaction conditions and the subtle difference among the catalyst structures [16, 37].


3. Photochemical CO2 reduction

In the preceding section, the electrocatalytic activities of the ruthenium complexes are introduced. The electrocatalyst can be utilized in photocatalytic systems by combining with a photosensitizer (PS). Figure 10 shows a schematic drawing of the photocatalytic system, in which the excited PS (PS*) receives an electron from an electron donor to afford the one-electron reduced PS (PS). The PS is the more powerful reagent than PS*, and it can inject an electron to the electrocatalyst. The catalyst can work similarly as the electroreduction occurs. In this section, the photocatalytic CO2 reduction by the ruthenium complexes is expounded.

Figure 10.

A schematic drawing of photocatalytic reduction by combining a photosensitizer (PS) with an electrocatalyst (Cat.).

3.1. Photosensitizer and sacrificial electron donors

The most common photosensitizer used in photocatalytic CO2 reduction is [Ru(bpy)3]2+ and the derivatives. Figure 11 shows the absorption and emission spectra of [Ru(bpy)3]2+ in acetonitrile. The complex exhibits an absorption band at 400–500 nm, which is assignable to metal-to-ligand charge transfer (MLCT). When excited at the band, the emission at the longer wavelengths is observed. The emission is not fluorescence but room-temperature phosphorescence, which is sensitive to O2. Therefore, the emission spectrum should be carefully measured under deaerated conditions [88]. The lifetime of the excited state of [Ru(bpy)3]2+ is 1.10 μs in acetonitrile [89, 90]. The quantum yield has been recently reevaluated as 0.095 in acetonitrile [91]. The oxidation potential (corresponding to the reducing ability) of the excited state (PS*) is −0.81 V vs. SCE (CH3CN), while this of the one-electron reduced species (PS) is −1.33 V. As the electrochemical CO2 reduction catalyzed by the ruthenium complexes proceeds under electrolysis at −1.30 V vs. SCE, it requires the reducing ability of PS. In general, the CO2 reduction requires higher energy than H2 production by reduction of H2O, and therefore, the photocatalytic CO2 reduction does not utilize the excited state but the one-electron reduced species.

Figure 11.

Absorption and emission (phosphorescence) spectra of [Ru(bpy)3]2+ in deaerated CH3CN at room temperature.

To generate the one-electron reduced species PS, the electron donors can reductively quench the excited state of the photosensitizer. As the reduction potential of the excited state of [Ru(bpy)3]2+ is +0.77 V vs. SCE (CH3CN), the electron donors which can be oxidized at less positive potentials than +0.77 V. Figure 12 shows the examples of the electron donors which are actually used in photocatalytic CO2 reduction [16, 92]. Ascorbate ion (AscH) can be used in aqueous solution, but amines (triethylamine (TEA) and triethanolamine (TEOA)) cannot work in the presence of water because they are protonated to afford the ammonium ions which cannot give an electron. 1-Benzyl-1,4-dihydronicotineamide (BNAH) is a model compound of NADH in nature. NADH is a two-electron donor and is oxidized to yield NAD+. However, the model compound BNAH cannot give two electrons in the oxidation by the excited state of [Ru(bpy)3]2+ but provides one electron to afford the dimer BNA2. 1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) and the derivatives (e.g., BI(OH)H), which have much stronger reducing power than BNAH, have been recently utilized in photocatalytic CO2 reduction. BIH provides two electrons to yield the oxidation product BI+.

Figure 12.

Examples of the electron donors (D) used in photochemical CO2 reduction.

These electron donors are called the sacrificial reagents because the one-electron oxidized species occur chemical changes or decompose so as to prevent back electron transfer. They are useful in order to investigate the reductive half reaction. However, from the viewpoint of the energy balance, the reduction-oxidation (redox) systems in which water is oxidized and CO2 is reduced are desired.

3.2. Photocatalytic CO2 reduction

Our group have investigated the photochemical CO2 reduction by the system consisting of trans(Cl)-[Ru(bpy)(CO)2Cl2], [Ru(bpy)3]2+ and BNAH as the catalyst, the photosensitizer, and the electron donor, respectively (Figure 13). The catalysis had been carried out in N,N-dimethylformamide (DMF)/water [57, 59]. However, it was indicated that hydration of DMF affording formate became a serious problem in quantifying formate [93]. We proposed the use of N,N-dimethylacetamide (DMA), of which the dehydration does not produce formate but acetate, instead of DMF [39]. Although the photocatalysis strongly depends on the solvent system, the reaction proceeds in DMA/water similarly as in DMF/water.

Figure 13.

Photochemical CO2 reduction catalyzed by trans(Cl)-[Ru(bpy)(CO)2Cl2] with [Ru(bpy)3]2+ (a photosensitizer (PS)) and BNAH (an electron donor).

The catalytic reaction proceeds by receiving electrons from the photochemically driven electron relay system. For two-electron reduction of CO2 to CO or HCOOH, the electron relay cycle has to go round two times when the catalytic cycle turns one time. The electron source is not an electrode, but the reaction had been supposed to proceed according to the same mechanism as in electrochemical reduction. However, it has been recently indicated that in some cases, the reaction mechanisms of the photochemical CO2 reduction are likely different from the electrochemical one [16]. For example, unusual catalyst concentration dependence on the product selectivity (CO/HCOO) in the photocatalysis has been observed: at high catalyst concentration the selectivity of HCOO increases [37]. To elucidate the peculiar catalyst concentration effect, the mechanisms as shown in the right cycle in Figure 14 are proposed. At the high concentration of the catalyst, the reduced catalyst forms a dimer, which is proposed to selectively afford HCOO. The dimer of the complex is similar as the intermediate of polymerization, but it is not detected in the photocatalytic system because the absorption spectrum cannot be conformed due to the overlapped absorption of [Ru(bpy)3]2+. Alternatively, the photocatalytic CO2 reduction by trans(Cl)-[Ru(6Mesbpy)(CO)2Cl2] which does not dimerize because of the steric hindrance of the ligand has been examined. The ruthenium complex selectively produces CO in the photochemical CO2 reduction, and it demonstrates that the dimerization of the catalyst relates on the HCOO production. It is suggested that the catalyst concentration dependence is not observed in a DMA/ethanol solution. It indicates that HCOO also produces in the cycle consisting of mono-nuclear ruthenium complexes as proposed for the electrocatalytic CO2 reduction (Figure 14, left cycle) [31].

Figure 14.

A proposed reaction mechanism for photocatalytic CO2 reduction by trans(Cl)-[Ru(bpy)(CO)2Cl2].

The photochemical CO2 reduction catalyzed by trans(Cl)-[Ru(2,2′-bipyridine)(CO)2Cl2] bearing two methyl groups at 4,4′- or 5,5′-positions in the ligand has been recently reported [64]. As the catalytic activities of these complexes at low catalyst concentrations are almost the same, the intrinsic activities are considered to be identical. However, the catalytic activities of these complexes are different at high catalyst concentration, where the rate-determining step is not in the catalytic cycle but in the electron relay cycle: the ruthenium complex with dimethyl groups at 5,5′-positions in the 2,2′-bipyridyl ligand is higher than that at 4,4′-positions. The efficiency of the back-electron transfer from the reduced catalyst to the photosensitizer is lower, or the cage escape yield for the sensitizer-catalyst complex is higher in the 5,5′-dimethyl complex than in the 4,4′-complex.

These phenomena have not been observed in electrochemical CO2 reduction. It is probably because that the homogenous photocatalytic CO2 reduction contains the diffusion process of the electron relay between the photosensitizer and the catalyst. The speed of the electron supply also sometimes affects the reaction mechanisms [16, 37].

3.3. Application to heterogeneous catalysts

Heterogeneous catalysts are industrially important because they are useful for separating the starting materials and the products from the catalyst and can be recovered and reused. The molecular catalysts can be utilized to develop the heterogeneous catalysts. For photocatalysts of CO2 reduction, combining the molecular catalysts with semiconductor [32, 94, 95], metal-organic frameworks (MOFs) [96, 97] or periodic mesoporous organosilicas (PMOs) [98, 99, 100, 101] are actively researched. We have also developed a novel PMO consisting of 2,2′-bipyridyl framework by introducing two different ruthenium complexes as a photosensitizing site (Ru(PS)) and a catalytic site (Ru(Cat)) as shown in Figure 15 [99]. Photochemical CO2 reduction by the PMO catalyst has catalytically produced CO and formate. The product selectivity (CO/formate) becomes large with increasing the ratio of Ru(PS) to Ru(Cat) (x/y). The photocatalysts can be recycled at least three times without losing the catalytic activity, demonstrating that the Ru(PS) and Ru(Cat) units are strongly immobilized on the BPy-PMO framework.

Figure 15.

Photocatalytic CO2 reduction by periodic mesoporous organosilica (PMO) containing two different ruthenium complexes as photosensitizing and catalytic sites.


4. Future prospects

The molecular catalysts are applicable to various photocatalytic systems. Ultimately, our goal is to construct an artificial photosynthetic system. An example is shown in Figure 16. In the system, the electrons are not supplied from the sacrificial electron donor but from water which is the same as in natural photosynthetic system. As the CO2 reduction requires a high potential, two photosensitizing systems would be combined as the Z-scheme mechanism in the natural photosynthesis. In order to realize the artificial photosynthesis, we have to overcome some problems. One is to perform these reactions (water oxidation, photo-induced electron transfer and CO2 reduction, etc.) under the similar conditions or in the separated circumstances. Another is to match the velocities among the reactions; even if the efficient catalyst for CO2 reduction was obtained, the speeds for the water oxidation and the electron supply have to match with that of CO2 reduction.

Figure 16.

A schematic drawing for an artificial photosynthetic system.

There would be many other problems to construct the artificial photosynthesis. However, the real system which can efficiently work has already existed in nature. We will realize it with a lot of ideas to overcome many problems one by one.



This work was supported by a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science and Technology (17K05815). This work was also supported by the PRESTO Program of JST, and a Grant-in-Aid for Scientific Research on Innovative Areas, “Artificial Photosynthesis (AnApple)” (No. 15H00882), from the Japan Society for the Promotion of Science (JSPS).


  1. 1. Olah GA, Prakash GKS, Goeppert A. Anthropogenic chemical carbon cycle for a sustainable future. Journal of the American Chemical Society. 2011;133:12881-12898. DOI: 10.1021/ja202642y
  2. 2. Arakawa H, Aresta M, Armor JN, Barteau MA, Beckman EJ, Bell AT, Bercaw JE, Creutz C, Dinjus E, Dixon DA, Domen K, DuBois DL, Eckert J, Fujita E, Gibson DH, Goddard WA, Goodman DW, Keller J, Kubas GJ, Kung HH, Lyons JE, Manzer LE, Marks TJ, Morokuma K, Nicholas KM, Periana R, Que L, Rostrup-Nielson J, Sachtler WMH, Schmidt LD, Sen A, Somorjai GA, Stair PC, Stults BR, Tumas W. Catalysis research of relevance to carbon management: Progress, challenges, and opportunities. Chemical Reviews. 2001;101:953-996. DOI: 10.1021/cr000018s
  3. 3. Mikkelsen M, Jorgensen M, Krebs FC. The teraton challenge. A review of fixation and transformation of carbon dioxide. Energy & Environmental Science. 2010;3:43-81. DOI: 10.1039/b912904a
  4. 4. Sakakura T, Choi J-C, Yasuda H. Transformation of carbon dioxide. Chemical Reviews. 2007;107:2365-2387. DOI: 10.1021/cr068357u
  5. 5. Schwarz HA, Dodson RW. Reduction potentials of CO2 and the alcohol radicals. The Journal of Physical Chemistry. 1989;93:409-414. DOI: 10.1021/j100338a079
  6. 6. Yamazaki Y, Takeda H, Ishitani O. Photocatalytic reduction of CO2 using metal complexes. Journal of Photochemistry and Photobiology C Photochemistry Reviews. 2015;25:106-137. DOI:
  7. 7. Hildebrandt D, Glasser D, Hausberger B, Patel B, Glasser BJ. Producing transportation fuels with less work. Science. 2009;323:1680-1681. DOI: 10.1126/science.1168455
  8. 8. Singh AK, Singh S, Kumar A. Hydrogen energy future with formic acid: A renewable chemical hydrogen storage system. Catalysis Science & Technology. 2016;6:12-40. DOI: 10.1039/c5cy01276g
  9. 9. DuBois DL. Development of molecular electrocatalysts for energy storage. Inorganic Chemistry. 2014;53:3935-3960. DOI: 10.1021/ic4026969
  10. 10. Appel AM, Bercaw JE, Bocarsly AB, Dobbek H, DuBois DL, Dupuis M, Ferry JG, Fujita E, Hille R, Kenis PJA, Kerfeld CA, Morris RH, Peden CHF, Portis AR, Ragsdale SW, Rauchfuss TB, Reek JNH, Seefeldt LC, Thauer RK, Waldrop GL. Frontiers, opportunities, and challenges in biochemical and chemical catalysis of CO2 fixation. Chemical Reviews. 2013;113:6621-6658. DOI: 10.1021/cr300463y
  11. 11. Schneider J, Jia H, Muckerman JT, Fujita E. Thermodynamics and kinetics of CO2, CO, and H+ binding to the metal centre of CO2 reductioncatalysts. Chemical Society Reviews. 2012;41:2036-2051. DOI: 10.1039/c1cs15278e
  12. 12. Windle CD, Perutz RN. Advances in molecular photocatalytic and electrocatalytic CO2 reduction. Coordination Chemistry Reviews. 2012;256:2562-2570. DOI: 10.1016/j.ccr.2012.03.010
  13. 13. Yui T, Tamaki Y, Sekizawa K, Ishitani O. Photocatalytic reduction of CO2: From molecules to semiconductors. Topics in Current Chemistry. 2011;303:151-184. DOI: 10.1007/128_2011_139
  14. 14. Takeda H, Ishitani O. Development of efficient photocatalytic systems for CO2 reduction using mononuclear and multinuclear metal complexes based on mechanistic studies. Coordination Chemistry Reviews. 2010;254:346-354. DOI: 10.1016/j.ccr.2009.09.030
  15. 15. Savéant J-M. Molecular catalysis of electrochemical reactions. Mechanistic aspects. Chemical Reviews. 2008;108:2348-2378. DOI: 10.1021/cr068079z
  16. 16. Kuramochi Y, Ishitani O, Ishida H. Reaction mechanisms of catalytic photochemical CO2 reduction using Re(I) and Ru(II) complexes. Coordination Chemistry Reviews. 2018. DOI: 10.1016/j.ccr.2017.11.023
  17. 17. Sampson MD, Nguyen AD, Grice KA, Moore CE, Rheingold AL, Kubiak CP. Manganese catalysts with bulky bipyridine ligands for the electrocatalytic reduction of carbon dioxide: Eliminating dimerization and altering catalysis. Journal of the American Chemical Society. 2014;136:5460-5471. DOI: 10.1021/ja501252f
  18. 18. Takeda H, Koizumi H, Okamoto K, Ishitani O. Photocatalytic CO2 reduction using a Mn complex as a catalyst. Chemical Communications. 2014;50:1491-1493. DOI: 10.1039/c3cc48122k
  19. 19. Bourrez M, Molton F, Chardon-Noblat S, Deronzier A. [Mn(bipyridyl)(CO)3Br]: An abundant metal carbonyl complex as efficient electrocatalyst for CO2 reduction. Angewandte Chemie, International Edition. 2011;50:9903-9906. DOI: 10.1002/anie.201103616
  20. 20. Costentin C, Drouet S, Robert M, Savéant J-M. A local proton source enhances CO2 electroreduction to CO by a molecular Fe catalyst. Science. 2012;338:90-94. DOI: 10.1126/science.1224581
  21. 21. Hammouche M, Lexa D, Momenteau M, Saveant JM. Chemical catalysis of electrochemical reactions. Homogeneous catalysis of the electrochemical reduction of carbon dioxide by iron(0) porphyrins. Role of the addition of magnesium cations. Journal of the American Chemical Society. 1991;113:8455-8466. DOI: 10.1021/ja00022a038
  22. 22. Matsuoka S, Yamamoto K, Ogata T, Kusaba M, Nakashima N, Fujita E, Yanagida S. Efficient and selective electron mediation of cobalt complexes with cyclam and related macrocycles in the p-terphenyl-catalyzed photoreduction of carbon dioxide. Journal of the American Chemical Society. 1993;115:601-609. DOI: 10.1021/ja00055a032
  23. 23. Ziessel R, Hawecker J, Lehn J-M. Photogeneration of carbon monoxide and of hydrogen via simultaneous photochemical reduction of carbon dioxide and water by visible-light irradiation of organic solutions containing Tris(2,2′-bipyridine)ruthenium(II) and cobalt(II) species as homogeneous catalysts. Helvetica Chimica Acta. 1986;69:1065-1084. DOI: 10.1002/hlca.19860690514
  24. 24. Tinnemans AHA, Koster TPM, Thewissen DHMW, Mackor A. Tetraaza-macrocyclic cobalt(II) and nickel(II) complexes as electron-transfer agents in the photo(electro)chemical and electrochemical reduction of carbon dioxide. Recueil des Travaux Chimiques des Pays-Bas. 1984;103:288-295. DOI: 10.1002/recl.19841031004
  25. 25. Rudolph M, Dautz S, Jäger E-G. Macrocyclic [N42−] coordinated nickel complexes as catalysts for the formation of oxalate by electrochemical reduction of carbon dioxide. Journal of the American Chemical Society. 2000;122:10821-10830. DOI: 10.1021/ja001254n
  26. 26. Grant JL, Goswami K, Spreer LO, Otvos JW, Calvin M. Photochemical reduction of carbon dioxide to carbon monoxide in water using a nickel(II) tetra-azamacrocycle complex as catalyst. Journal of the Chemical Society, Dalton Transactions. 1987:2105-2109. DOI: 10.1039/dt9870002105
  27. 27. Beley M, Collin JP, Ruppert R, Sauvage JP. Electrocatalytic reduction of carbon dioxide by nickel cyclam2+ in water: Study of the factors affecting the efficiency and the selectivity of the process. Journal of the American Chemical Society. 1986;108:7461-7467. DOI: 10.1021/ja00284a003
  28. 28. Beley M, Collin J-P, Ruppert R, Sauvage J-P. Nickel(II)-cyclam: An extremely selective electrocatalyst for reduction of CO2 in water. Journal of the Chemical Society, Chemical Communications. 1984:1315-1316. DOI: 10.1039/c39840001315
  29. 29. Angamuthu R, Byers P, Lutz M, Spek AL, Bouwman E. Electrocatalytic CO2 conversion to oxalate by a copper complex. Science. 2010;327:313-315. DOI: 10.1126/science.1177981
  30. 30. Clark ML, Grice KA, Moore CE, Rheingold AL, Kubiak CP. Electrocatalytic CO2 reduction by M(bpy-R)(CO)4 (M = Mo, W; R = H, tBu) complexes. Electrochemical, spectroscopic, and computational studies and comparison with group 7 catalysts. Chemical Science. 2014;5:1894-1900. DOI: 10.1039/c3sc53470g
  31. 31. Ishida H, Sakaba A. Temperature dependence of photocatalytic CO2 reduction by trans(Cl)-Ru(bpy)(CO)2Cl2: Activation energy difference between CO and formate production. Faraday Discussions. 2017;198:263-277. DOI: 10.1039/c6fd00242k
  32. 32. Kuriki R, Matsunaga H, Nakashima T, Wada K, Yamakata A, Ishitani O, Maeda K. Nature-inspired, highly durable CO2 reduction system consisting of a binuclear ruthenium(II) complex and an organic semiconductor using visible light. Journal of the American Chemical Society. 2016;138:5159-5170. DOI: 10.1021/jacs.6b01997
  33. 33. Kuriki R, Ishitani O, Maeda K. Unique solvent effects on visible-light CO2 reduction over ruthenium(II)-complex/carbon nitride hybrid photocatalysts. ACS Applied Materials & Interfaces. 2016;8:6011-6018. DOI: 10.1021/acsami.5b11836
  34. 34. Yoshitomi F, Sekizawa K, Maeda K, Ishitani O. Selective formic acid production via CO2 reduction with visible light using a hybrid of a perovskite tantalum oxynitride and a binuclear ruthenium(II) complex. ACS Applied Materials & Interfaces. 2015;7:13092-13097. DOI: 10.1021/acsami.5b03509
  35. 35. Machan CW, Sampson MD, Kubiak CP. A molecular ruthenium electrocatalyst for the reduction of carbon dioxide to CO and Formate. Journal of the American Chemical Society. 2015;137:8564-8571. DOI: 10.1021/jacs.5b03913
  36. 36. Kuriki R, Sekizawa K, Ishitani O, Maeda K. Visible-light-driven CO2 reduction with carbon nitride: Enhancing the activity of ruthenium catalysts. Angewandte Chemie, International Edition. 2015;54:2406-2409. DOI: 10.1002/anie.201411170
  37. 37. Kuramochi Y, Itabashi J, Fukaya K, Enomoto A, Yoshida M, Ishida H. Unexpected effect of catalyst concentration on photochemical CO2 reduction by trans(Cl)-Ru(bpy)(CO)2Cl2: New mechanistic insight into the CO/HCOO selectivity. Chemical Science. 2015;6:3063-3074. DOI: 10.1039/c5sc00199d
  38. 38. Kuramochi Y, Fukaya K, Yoshida M, Ishida H. Trans(Cl)-[Ru(5,5′-diamide-2,2′-bipyridine)(CO)2Cl2]: Synthesis, structure, and photocatalytic CO2 reduction activity. Chemistry—A European Journal. 2015;21:10049-10060. DOI: 10.1002/chem.201500782
  39. 39. Kuramochi Y, Kamiya M, Ishida H. Photocatalytic CO2 reduction in N,N-dimethylacetamide/water as an alternative solvent system. Inorganic Chemistry. 2014;53:3326-3332. DOI: 10.1021/ic500050q
  40. 40. Maeda K, Kuriki R, Zhang M, Wang X, Ishitani O. The effect of the pore-wall structure of carbon nitride on photocatalytic CO2 reduction under visible light. Journal of Materials Chemistry A. 2014;2:15146-15151. DOI: 10.1039/c4ta03128h
  41. 41. Maeda K, Sekizawa K, Ishitani O. A polymeric-semiconductor-metal-complex hybrid photocatalyst for visible-light CO2 reduction. Chemical Communications. 2013;49:10127-10129. DOI: 10.1039/c3cc45532g
  42. 42. Tamaki Y, Morimoto T, Koike K, Ishitani O. Photocatalytic CO2 reduction with high turnover frequency and selectivity of formic acid formation using Ru(II) multinuclear complexes. Proceedings of the National Academy of Sciences of the United States of America. 2012;109:15673-15678. DOI: 10.1073/pnas.1118336109
  43. 43. Yamanaka K-I, Sato S, Iwaki M, Kajino T, Morikawa T. Photoinduced electron transfer from nitrogen-doped tantalum oxide to adsorbed ruthenium complex. Journal of Physical Chemistry C. 2011;115:18348-18353. DOI: 10.1021/jp205223k
  44. 44. Suzuki TM, Tanaka H, Morikawa T, Iwaki M, Sato S, Saeki S, Inoue M, Kajino T, Motohiro T. Direct assembly synthesis of metal complex-semiconductor hybrid photocatalysts anchored by phosphonate for highly efficient CO2 reduction. Chemical Communications. 2011;47:8673-8675. DOI: 10.1039/c1cc12491a
  45. 45. Sato S, Arai T, Morikawa T, Uemura K, Suzuki TM, Tanaka H, Kajino T. Selective CO2 conversion to Formate conjugated with H2O oxidation utilizing semiconductor/complex hybrid photocatalysts. Journal of the American Chemical Society. 2011;133:15240-15243. DOI: 10.1021/ja204881d
  46. 46. Arai T, Tajima S, Sato S, Uemura K, Morikawa T, Kajino T. Selective CO2 conversion to formate in water using a CZTS photocathode modified with a ruthenium complex polymer. Chemical Communications. 2011;47:12664-12666. DOI: 10.1039/c1cc16160a
  47. 47. Sato S, Morikawa T, Saeki S, Kajino T, Motohiro T. Visible-light-induced selective CO2 reduction utilizing a ruthenium complex electrocatalyst linked to a p-type nitrogen-doped Ta2O5 semiconductor. Angewandte Chemie International Edition in English. 2010;49:5101-5105. S5101/5101-S5101/5111. DOI: 10.1002/anie.201000613
  48. 48. Arai T, Sato S, Uemura K, Morikawa T, Kajino T, Motohiro T. Photoelectrochemical reduction of CO2 in water under visible-light irradiation by a p-type InP photocathode modified with an electropolymerized ruthenium complex. Chemical Communications. 2010;46:6944-6946. DOI: 10.1039/c0cc02061c
  49. 49. Creutz C, Chou MH. Rapid transfer of hydride ion from a ruthenium complex to C1 species in water. Journal of the American Chemical Society. 2007;129:10108-10109. DOI: 10.1021/ja074158w
  50. 50. Chardon-Noblat S, Deronzier A, Ziessel R, Zsoldos D. Electroreduction of CO2 catalyzed by polymeric [Ru(bpy)(CO)2]n films in aqueous media: Parameters influencing the reaction selectivity. Journal of Electroanalytical Chemistry. 1998;444:253-260. DOI: 10.1016/s0022-0728(97)00584-6
  51. 51. Chardon-Noblat S, Deronzier A, Ziessel R, Zsoldos D. Selective synthesis and electrochemical behavior of trans(Cl)- and cis(Cl)-[Ru(bpy)(CO)2Cl2] complexes (bpy = 2,2'-Bipyridine). Comparative studies of their electrocatalytic activity toward the reduction of carbon dioxide. Inorganic Chemistry. 1997;36:5384-5389. DOI: 10.1021/ic9701975
  52. 52. Nagao H, Mizukawa T, Tanaka K. Carbon-carbon bond formation in the electrochemical reduction of carbon dioxide catalyzed by a ruthenium complex. Inorganic Chemistry. 1994;33:3415-3420. DOI: 10.1021/ic00093a033
  53. 53. Collomb-Dunand-Sauthier M-N, Deronzier A, Ziessel R. Electrocatalytic reduction of carbon dioxide with mono(bipyridine)carbonylruthenium complexes in solution or as polymeric thin films. Inorganic Chemistry. 1994;33:2961-2967. DOI: 10.1021/ic00091a040
  54. 54. Collomb-Dunand-Sauthier MN, Deronzier A, Ziessel R. Electrocatalytic reduction of CO2 in water on a polymeric [{Ru0(bpy)(CO)2}n] (bpy = 2,2′-bipyridine) complex immobilized on carbon electrodes. Journal of the Chemical Society, Chemical Communications. 1994:189-191. DOI: 10.1039/c39940000189
  55. 55. Chardon-Noblat S, Collomb-Dunand-Sauthier MN, Deronzier A, Ziessel R, Zsoldos D. Formation of polymeric [{Ru0(bpy)(CO)2}n] films by electrochemical reduction of [Ru(bpy)2(CO)2](PF6)2: Its implication in CO2 electrocatalytic reduction. Inorganic Chemistry. 1994;33:4410-4412. DOI: 10.1021/ic00097a034
  56. 56. Lehn JM, Ziessel R. Photochemical reduction of carbon dioxide to formate catalyzed by (2,2′-bipyridine)- or (1,10-phenanthroline)ruthenium(II) complexes. Journal of Organometallic Chemistry. 1990;382:157-173. DOI: 10.1016/0022-328x(90)85224-m
  57. 57. Ishida H, Terada T, Tanaka K, Tanaka T. Photochemical carbon dioxide reduction catalyzed by bis(2,2′-bipyridine)dicarbonylruthenium(2+) using triethanolamine and 1-benzyl-1,4-dihydronicotinamide as an electron donor. Inorganic Chemistry. 1990;29:905-911. DOI: 10.1021/ic00330a004
  58. 58. Ishida H, Fujiki K, Ohba T, Ohkubo K, Tanaka K, Terada T, Tanaka T. Ligand effects of ruthenium 2,2′-bipyridine and 1,10-phenanthroline complexes on the electrochemical reduction of carbon dioxide. Journal of the Chemical Society, Dalton Transactions. 1990:2155-2160. DOI: 10.1039/DT9900002155
  59. 59. Ishida H, Tanaka K, Tanaka T. Photochemical carbon dioxide reduction by an NADH model compound in the presence of ruthenium complexes [Ru(bpy)3]2+ and [Ru(bpy)2(CO)2]2+ (bpy = 2,2′-bipyridine) in water/DMF. Chemistry Letters. 1988;17:339-342. DOI: 10.1246/cl.1988.339
  60. 60. Ishida H, Tanaka K, Tanaka T. Photoreduction of carbon dioxide in the [Ru(bpy)2(CO)2]2+ (bpy = 2,2′-bipyridine)/[Ru(bpy)3]2+ or [Ru(phen)3]2+ (phen = phenanthroline)/triethanolamine/N,N-dimethylformamide system. Chemistry Letters. 1987;16:1035-1038. DOI: 10.1246/cl.1987.1035
  61. 61. Ishida H, Tanaka H, Tanaka K, Tanaka T. Selective formation of formate in the electrochemical carbon dioxide reduction catalyzed by [Ru(bpy)2(CO)2]2+ (bpy = 2,2′-bipyridine). Journal of the Chemical Society, Chemical Communications. 1987:131-132. DOI: 10.1039/c39870000131
  62. 62. Ishida H, Tanaka K, Tanaka T. Electrochemical CO2 reduction catalyzed by ruthenium complexes [Ru(bpy)2(CO)2]2+ and [Ru(bpy)2(CO)Cl]+. Effect of pH on the formation of CO and HCOO. Organometallics. 1987;6:181-186. DOI: 10.1021/om00144a033
  63. 63. Ishida H, Tanaka K, Tanaka T. The electrochemical reduction of carbon dioxide catalyzed by ruthenium carbonyl complexes. Chemistry Letters. 1985;14:405-406. DOI: 10.1246/cl.1985.405
  64. 64. Kuramochi Y, Itabashi J, Toyama M, Ishida H. Photochemical CO2 reduction catalyzed by trans(Cl)-Ru(2,2′-bipyridine)(CO)2Cl2 bearing two methyl groups at 4,4′-, 5,5′- or 6,6'-positions in the ligand. ChemPhotoChem. 2018;2:314-322. DOI: 10.1002/cptc.201700201
  65. 65. Rasmussen SC, Richter MM, Yi E, Place H, Brewer KJ. Synthesis and characterization of a series of novel rhodium and iridium complexes containing polypyridyl bridging ligands: Potential uses in the development of multimetal catalysts for carbon dioxide reduction. Inorganic Chemistry. 1990;29:3926-3932. DOI: 10.1021/ic00345a005
  66. 66. Bolinger CM, Story N, Sullivan BP, Meyer TJ. Electrocatalytic reduction of carbon dioxide by 2,2′-bipyridine complexes of rhodium and iridium. Inorganic Chemistry. 1988;27:4582-4587. DOI: 10.1021/ic00298a016
  67. 67. Bernatis PR, Miedaner A, Haltiwanger RC, DuBois DL. Exclusion of six-coordinate intermediates in the electrochemical reduction of CO2 catalyzed by [Pd(triphosphine)(CH3CN)](BF4)2 complexes. Organometallics. 1994;13:4835-4843. DOI: 10.1021/om00024a029
  68. 68. DuBois DL, Miedaner A. Mediated electrochemical reduction of CO2. Preparation and comparison of an isoelectronic series of complexes. Journal of the American Chemical Society. 1987;109:113-117. DOI: 10.1021/ja00235a019
  69. 69. Riplinger C, Sampson MD, Ritzmann AM, Kubiak CP, Carter EA. Mechanistic contrasts between manganese and rhenium bipyridine electrocatalysts for the reduction of carbon dioxide. Journal of the American Chemical Society. 2014;136:16285-16298. DOI: 10.1021/ja508192y
  70. 70. Kou Y, Nabetani Y, Masui D, Shimada T, Takagi S, Tachibana H, Inoue H. Direct detection of key reaction intermediates in photochemical CO2 reduction sensitized by a rhenium bipyridine complex. Journal of the American Chemical Society. 2014;136:6021-6030. DOI: 10.1021/ja500403e
  71. 71. Morimoto T, Nishiura C, Tanaka M, Rohacova J, Nakagawa Y, Funada Y, Koike K, Yamamoto Y, Shishido S, Kojima T, Saeki T, Ozeki T, Ishitani O. Ring-shaped Re(I) multinuclear complexes with unique photofunctional properties. Journal of the American Chemical Society. 2013;135:13266-13269. DOI: 10.1021/ja406144h
  72. 72. Tamaki Y, Koike K, Morimoto T, Yamazaki Y, Ishitani O. Red-light-driven Photocatalytic reduction of CO2 using Os(II)–re(I) supramolecular complexes. Inorganic Chemistry. 2013;52:11902-11909. DOI: 10.1021/ic4015543
  73. 73. Agarwal J, Fujita E, Schaefer HF, Muckerman JT. Mechanisms for CO production from CO2 using reduced rhenium tricarbonyl catalysts. Journal of the American Chemical Society. 2012;134:5180-5186. DOI: 10.1021/ja2105834
  74. 74. Takeda H, Koike K, Inoue H, Ishitani O. Development of an efficient photocatalytic system for CO2 reduction using rhenium(I) complexes based on mechanistic studies. Journal of the American Chemical Society. 2008;130:2023-2031. DOI: 10.1021/ja077752e
  75. 75. Gholamkhass B, Mametsuka H, Koike K, Tanabe T, Furue M, Ishitani O. Architecture of supramolecular metal complexes for photocatalytic CO2 reduction: Ruthenium-rhenium bi- and tetranuclear complexes. Inorganic Chemistry. 2005;44:2326-2336. DOI: 10.1021/ic048779r
  76. 76. Hawecker J, Lehn JM, Ziessel R. Photochemical and electrochemical reduction of carbon dioxide to carbon monoxide mediated by (2,2′-bipyridine)tricarbonylchlororhenium(I) and related complexes as homogeneous catalysts. Helvetica Chimica Acta. 1986;69:1990-2012. DOI: 10.1002/hlca.19860690824
  77. 77. Chauvin J, Lafolet F, Chardon-Noblat S, Deronzier A, Jakonen M, Haukka M. Towards new molecular photocatalysts for CO2 reduction: Photo-induced electron transfer versus CO dissociation within [Os(NN)(CO)2Cl2] complexes. Chemistry—A European Journal. 2011;17:4313-4322. DOI: 10.1002/chem.201003098
  78. 78. Bruce MRM, Megehee E, Sullivan BP, Thorp HH, O'Toole TR, Downard A, Pugh JR, Meyer TJ. Electrocatalytic reduction of carbon dioxide based on 2,2′-bipyridyl complexes of osmium. Inorganic Chemistry. 1992;31:4864-4873. DOI: 10.1021/ic00049a027
  79. 79. Sato S, Morikawa T, Kajino T, Ishitani O. A highly efficient mononuclear iridium complex photocatalyst for CO2 reduction under visible light. Angewandte Chemie International Edition In English. 2013;52:988-992. DOI: 10.1002/anie.201206137
  80. 80. Kang P, Cheng C, Chen Z, Schauer CK, Meyer TJ, Brookhart M. Selective electrocatalytic reduction of CO2 to formate by water-stable iridium dihydride pincer complexes. Journal of the American Chemical Society. 2012;134:5500-5503. DOI: 10.1021/ja300543s
  81. 81. Costentin C, Passard G, Savéant J-M. Benchmarking of homogeneous electrocatalysts: Overpotential, turnover frequency, limiting turnover number. Journal of the American Chemical Society. 2015;137:5461-5467. DOI: 10.1021/jacs.5b00914
  82. 82. Mukhopadhyay TK, MacLean NL, Gan L, Ashley DC, Groy TL, Baik M-H, Jones AK, Trovitch RJ. Carbon dioxide promoted H+ reduction using a bis(imino)pyridine manganese electrocatalyst. Inorganic Chemistry. 2015;54:4475-4482. DOI: 10.1021/acs.inorgchem.5b00315
  83. 83. Tanaka H, Tzeng BC, Nagao H, Peng SM, Tanaka K. Comparative study on crystal structures of ruthenium bipyridine carbonyl complexes [Ru(bpy)2(CO)2](PF6)2, [Ru(bpy)2(CO)(C(O)OCH3)]B(C6H5)4∙CH3CN, and [Ru(bpy)2(CO)(η1-CO2)]∙3H2O (bpy = 2,2′-bipyridyl). Inorganic Chemistry. 1993;32:1508-1512. DOI: 10.1021/ic00060a029
  84. 84. Hitoshi I, Hiroaki T, Koji T, Toshio T. Electrochemical reaction of CO2 with Me2NH to afford N,N-dimethylformamide, catalyzed by [Ru(bpy)2(CO)2]2+ (bpy = 2,2′-bipyridine). Chemistry Letters. 1987;16:597-600. DOI: 10.1246/cl.1987.597
  85. 85. Pugh JR, Bruce MRM, Sullivan BP, Meyer TJ. Formation of a metal-hydride bond and the insertion of carbon dioxide. Key steps in the electrocatalytic reduction of carbon dioxide to formate anion. Inorganic Chemistry. 1991;30:86-91. DOI: 10.1021/ic00001a016
  86. 86. Chardon-Noblat S, Deronzier A, Zsoldos D, Ziessel R, Haukka M, Pakkanen T, Venalainen T. Mode of formation of polymer [{Ru(bipy)(CO)2}n] (bipy = 2,2′-bipyridine) films. Journal of the Chemical Society, Dalton Transactions. 1996:2581-2583
  87. 87. Masciocchi N, Sironi A, Chardon-Noblat S, Deronzier A. X-ray powder diffraction study of organometallic polymers: [Ru(L)(CO)2]n (L = 2,2‘-bipyridine or 1,10-phenanthroline). Organometallics. 2002;21:4009-4012. DOI: 10.1021/om020298x
  88. 88. Ishida H, Bünzli J-C, Beeby A. Guidelines for measurement of luminescence spectra and quantum yields of inorganic and organometallic compounds in solution and solid state (IUPAC technical report). Pure and Applied Chemistry. 2016;88:701. DOI: 10.1515/pac-2014-0706
  89. 89. Kalyanasundaram K. Photophysics, photochemistry and solar energy conversion with tris(bipyridyl)ruthenium(II) and its analogues. Coordination Chemistry Reviews. 1982;46:159-244. DOI: 10.1016/0010-8545(82)85003-0
  90. 90. Juris A, Balzani V, Barigelletti F, Campagna S, Belser P, von Zelewsky A. Ru(II) polypyridine complexes: Photophysics, photochemistry, eletrochemistry, and chemiluminescence. Coordination Chemistry Reviews. 1988;84:85-277. DOI: 10.1016/0010-8545(88)80032-8
  91. 91. Suzuki K, Kobayashi A, Kaneko S, Takehira K, Yoshihara T, Ishida H, Shiina Y, Oishi S, Tobita S. Reevaluation of absolute luminescence quantum yields of standard solutions using a spectrometer with an integrating sphere and a back-thinned CCD detector. Physical Chemistry Chemical Physics. 2009;11:9850-9860. DOI: 10.1039/b912178a
  92. 92. Tamaki Y, Ishitani O. Supramolecular photocatalysts for the reduction of CO2. ACS Catalysis. 2017;7:3394-3409. DOI: 10.1021/acscatal.7b00440
  93. 93. Paul A, Connolly D, Schulz M, Pryce MT, Vos JG. Effect of water during the quantitation of formate in photocatalytic studies on CO2 reduction in dimethylformamide. Inorganic Chemistry. 2012;51:1977-1979. DOI: 10.1021/ic202121s
  94. 94. Sekizawa K, Maeda K, Domen K, Koike K, Ishitani O. Artificial Z-scheme constructed with a supramolecular metal complex and semiconductor for the photocatalytic reduction of CO2. Journal of the American Chemical Society. 2013;135:4596-4599. DOI: 10.1021/ja311541a
  95. 95. Nakada A, Nakashima T, Sekizawa K, Maeda K, Ishitani O. Visible-light-driven CO2 reduction on a hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON in aqueous solutions. Chemical Science. 2016;7:4364-4371. DOI: 10.1039/c6sc00586a
  96. 96. Kajiwara T, Fujii M, Tsujimoto M, Kobayashi K, Higuchi M, Tanaka K, Kitagawa S. Photochemical reduction of low concentrations of CO2 in a porous coordination polymer with a ruthenium(II)–CO complex. Angewandte Chemie, International Edition. 2016;55:2697-2700. DOI: 10.1002/anie.201508941
  97. 97. Sun D, Gao Y, Fu J, Zeng X, Chen Z, Li Z. Construction of a supported Ru complex on bifunctional MOF-253 for photocatalytic CO2 reduction under visible light. Chemical Communications. 2015;51:2645-2648. DOI: 10.1039/c4cc09797a
  98. 98. Wang X, Thiel I, Fedorov A, Coperet C, Mougel V, Fontecave M. Site-isolated manganese carbonyl on bipyridine-functionalities of periodic mesoporous organosilicas: Efficient CO2 photoreduction and detection of key reaction intermediates. Chemical Science. 2017;8:8204-8213. DOI: 10.1039/c7sc03512h
  99. 99. Kuramochi Y, Sekine M, Kitamura K, Maegawa Y, Goto Y, Shirai S, Inagaki S, Ishida H. Photocatalytic CO2 reduction by periodic mesoporous organosilica (PMO) containing two different ruthenium complexes as photosensitizing and catalytic sites. Chemistry—A European Journal. 2017;23:10301-10309. DOI: 10.1002/chem.201701466
  100. 100. Ueda Y, Takeda H, Yui T, Koike K, Goto Y, Inagaki S, Ishitani O. A visible-light harvesting system for CO2 reduction using a RuII-ReI photocatalyst adsorbed in mesoporous organosilica. ChemSusChem. 2015;8:439-442. DOI: 10.1002/cssc.201403194
  101. 101. Takeda H, Ohashi M, Tani T, Ishitani O, Inagaki S. Enhanced photocatalysis of rhenium(I) complex by light-harvesting periodic mesoporous organosilica. Inorganic Chemistry. 2010;49:4554-4559. DOI: 10.1021/ic1000914

Written By

Hitoshi Ishida

Submitted: 30 April 2017 Reviewed: 12 February 2018 Published: 21 March 2018